There has been used an image pickup apparatus configured such that in an image pickup device in which pixels are arranged in a two-dimensional matrix, image signals are output from the pixels in every row at a time, the image signals corresponding to one row are sequentially subjected to analog-to-digital conversion, and resultant signals are output as digital image signals in the past. To meet a recent demand of speedup of image signal output, there have been proposed systems for speeding up analog-to-digital conversion by arranging an analog-to-digital converter in each pixel and simultaneously performing analog-to-digital conversion in all pixels. For example, there is proposed a system configured such that an analog-to-digital converter having a comparison section that compares an analog image signal generated by a photodiode with a reference signal and a latch that retains a digital signal generated on the basis of a comparison result is arranged in each pixel. The reference signal means herein a signal at a voltage changing in a ramp state and a signal commonly input to the analog-to-digital converters corresponding to all the pixels arranged in the image pickup apparatus. In the system, the comparison section compares the analog image signal with the reference signal. In addition, when the voltage of the reference signal transitions from a lower state than a voltage of the analog image signal to a higher state or from the higher state to the lower state, the comparison section detects a change of this voltage and outputs the change of the voltage as the comparison result. Furthermore, a codeword that is a digital signal corresponding to the voltage of the reference signal is input to the latch, and the input codeword is retained in the latch on the basis of a detection result of the comparison section. Subsequently, the codeword retained in the latch is output as a result of analog-to-digital conversion (refer to, for example, NPL 1).